zic: cherrypick support for %z in time zone formats

zic: cherrypick support for %z in time zone formats

This extends the zic input format to add support for %z, which
expands to a UTC offset in as-short-as-possible ISO 8601 format.
It's intended to better support zones that do not have an established
abbreviation already.  tzdata2024b and higher require a version of
zic that supports the %z format.  From upstream tzcode.  OK beck@